It turns out antibacterial soaps aren’t so “clean” after all. A common chemical in antibacterial products, triclosan — which can be found soaps, toothpastes and mouthwashes — was found to impair muscle function in lab and animal tests.

Originally, the chemical, developed in the 1960s, was used in hospitals to prevent bacterial infections. Since then, it’s been used in countless household products, and several studies — mostly in animals — have hinted that the effects of triclosan may not be entirely beneficial.

Studies have shown that the chemical can disrupt the endocrine systems of several different animals, binding to receptor sites in the body, which prevents the thyroid hormone from functioning normally. Additionally, triclosan penetrates the skin and enters the bloodstream more easily than previously thought, and has turned up everywhere from aquatic environments to human breast milk in troubling quantities.

Now, in a new study published in the Proceedings of the National Academy of Sciences, researchers from the University of California, Davis, found that triclosan also interferes with muscle function. In the lab, they exposed human muscle cells, from the heart and elsewhere, to triclosan and discovered that the chemical interrupted cellular communication necessary for muscle contraction. Then the researchers exposed mice and fathead minnows to the chemical to see what would happen: after a single dose, the exposed mice showed 25% reduced heart muscle function and 18% reduced grip strength. In the fish, which were exposed to as much triclosan as would be expected in a week in the wild, the chemical led to poor performance in swimming tests that simulated escape from a predator.

For its part, the U.S. Food and Drug Administration (FDA) has not found the chemical hazardous to humans, but is in the process of reviewing the safety of products containing triclosan; those findings are expected at the end of the year. The FDA notes further that there’s no evidence suggesting that antibacterial soaps containing triclosan offer any additional health benefits over regular soap.